Container with crack-resistant rim

ABSTRACT

A plastic container comprises a bottom wall, a side wall extending upwardly and laterally outwardly from the bottom wall, and an undulating rim extending laterally outwardly from an upper portion of the side wall. The undulating rim forms a plurality of waves extending at least partially around the upper portion of the side wall. Each wave forms a crest and a trough that are generally perpendicular to an adjacent section of the upper portion of the side wall. In response to exerting a tensile load on the rim, the undulating rim acts like an accordion or spring and effectively increases the perimeter along an outer trimmed edge of the rim. In particular, the waves allow the rim to flatten, thereby reducing the hoop stress generated by the tensile load. By relieving the hoop stress on the outer trimmed edge of the rim, the tendency of the rim to crack or break is minimized.

FIELD OF THE INVENTION

[0001] The present invention relates generally to plastic containers and, more particularly, relates to a plastic container having a crack-resistant flexible rim that can withstand greater tensile loading before failure than prior plastic containers.

BACKGROUND OF THE INVENTION

[0002] Disposable plastic containers are often used by consumers to hold a variety of food products for consumption. The containers are typically in the form of a plate, tray, or bowl. In any of these forms, the plastic container includes a bottom wall, a continuous side wall extending upwardly and outwardly from the bottom wall, and a continuous rim extending laterally outwardly from an upper portion of the side wall. The continuous side wall of a bowl is greater in height than the continuous side wall of a plate or tray.

[0003] When food products are loaded into a plastic container and the container is handled by a consumer, tensile loads may be exerted onto the rim of the container. Such tensile loads can cause the rim to flex and possibly crack or break along its outermost “trimmed” edge at the point of tensile loading. If the tensile loading is prolonged, such cracks or breaks can be propagated inward from the outermost trimmed edge to worsen the problem. A cracked or broken rim can make it difficult to handle the container and can cause the container to fail during normal functional use.

SUMMARY OF THE INVENTION

[0004] Accordingly, an object of the present invention is to provide a plastic container having a crack-resistant flexible rim that can withstand greater tensile loading before failure than prior plastic containers.

[0005] This and other objects are realized by providing a plastic container comprising a bottom wall, a side wall extending upwardly and laterally outwardly from the bottom wall, and an undulating rim extending laterally outwardly from an upper portion of the side wall. The undulating rim forms a plurality of waves extending at least partially around the upper portion of the side wall. Each wave forms a crest and a trough that are generally perpendicular to an adjacent section of the upper portion of the side wall.

[0006] In response to exerting a tensile load on the rim, the undulating rim acts like an accordion or spring and effectively increases the perimeter along an outer trimmed edge of the rim. In particular, the waves allow the rim to flatten, thereby reducing the hoop stress generated by the tensile load. By relieving the hoop stress on the outer trimmed edge of the rim, the tendency of the rim to crack or break is minimized.

[0007] The above summary of the present invention is not intended to represent each embodiment, or every aspect of the present invention. This is the purpose of the figures and detailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

[0009]FIG. 1 is an isometric view of a plastic container embodying the present invention;

[0010]FIG. 2 is a top view of the plastic container at rest;

[0011]FIG. 3 is a side view of the plastic container at rest;

[0012]FIG. 4 is a magnified view of the area 4 in FIG. 3;

[0013]FIG. 5 is a side view of the plastic container under deflection;

[0014]FIG. 6 is a magnified view of the area 6 in FIG. 5; and

[0015]FIGS. 7a, 7 b, and 7 c are graphs showing the percentage increase of the circumference of deflected containers along their outer trimmed edge for different diameters, amplitudes and wave spacings.

[0016] While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Turning now to the drawings, FIGS. 1 and 2 depict a plastic container 10 embodying the present invention. The container 10 is preferably composed of expanded polystyrene foam having a thickness ranging from about 30 mils to about 100 mils, but may alternatively be composed of other polymeric materials such as oriented polystyrene, polyethylene terephthalate, polypropylene, and polyvinyl chloride. Further, although the container 10 is illustrated as being round in shape, the container 10 may alternatively be oval, rectangular, polygonal, or some other shape.

[0018] The plastic container 10 comprises a bottom wall 12, a continuous side wall 14 extending upwardly and laterally outwardly from the bottom wall 12, and a continuous flexible rim 16 extending laterally outwardly from an upper portion 18 of the side wall 14. As best shown in FIG. 3, the upper portion 18 of the side wall 14 is slightly arch-shaped and extends laterally outwardly from a remainder of the side wall 14. Depending upon the height of the side wall 14, the container 10 may be in the form of a bowl, plate, or tray. If the container 10 is in the form of a plate as illustrated, the side wall 14 is greater in height than the side wall of a tray but less in height than the side wall of a bowl.

[0019] The flexible rim 16 is designed to exhibit greater crack-resistance and to withstand greater tensile loading before failure than prior plastic containers. To provide such crack-resistance and resistance to tensile loading, the rim 16 forms periodic waves 20, i.e., undulations or ridges, encompassing the upper portion 18 of the side wall 14.

[0020] The waves 20 are immediately adjacent to each other and repeat in a regular pattern along the entire circumference of the container 10. That is, the waves 20 occupy the entire rim 16. Also, as shown in FIG. 2, each wave 20 occupies a circumferential angle α ranging from about 2 degrees to about 8 degrees, and, most preferably, about 5 degrees. This circumferential angle α is referred to herein as the “wave spacing.” Alternatively, the waves 20 may be divided into groups of one or more waves 20 that are slightly spaced from each other and form a regular or irregular pattern along the circumference of the container 10. The portions of the circumference between the groups of waves 20 may be non-wavy or flat. To avoid compromising the crack-resistance of the rim 16 proximate these non-wavy portions, the non-wavy portions preferably have a width no greater than about the width of a small human thumb, e.g., about 0.75 inches.

[0021] Referring to FIGS. 3 and 4, each wave 20 forms a crest 20 a and a trough 20 b that are generally perpendicular to an adjacent section of the upper portion 18 of the side wall 14. Although the crest 20 a and the trough 20 b of each wave 20 are shown in FIGS. 3 and 4 as being somewhat rounded, they may alternatively be more pointed in shape. When the plastic container 10 is at rest, the amplitude (height) H, the wave spacing α (FIG. 2), and the wavelength λ of the waves 20 is constant. The amplitude H is the vertical distance between the crest 20 a and the trough 20 b. The wave spacing α (FIG. 2) is the circumferential angle between adjacent crests 20 a or between adjacent troughs 20 b. The wavelength λ is determined from the wave spacing α (FIG. 2) and the diameter D of the container as follows:

λ=α/360°×π×D

[0022] where (π×D) is the circumference and (α/360°) is the percentage of the circumference occupied by each wave 20. A segment length L is the distance along the surface of the wave 20 between a crest 20 a and an adjacent trough 20 b and is determined from the amplitude H and wavelength λ as follows:

L={square root}{square root over (H²+(0.5×λ)²)}

[0023] Referring to FIGS. 5 and 6, in response to exerting a tensile load on the undulating rim 16, the rim 16 acts like an accordion or spring and effectively increases the circumference along an outer trimmed edge of the rim 16. In particular, the waves 20 allow the rim 16 to flatten, thereby reducing the hoop stress generated by the tensile load. By relieving the hoop stress on the outer trimmed edge of the rim, the tendency of the rim to crack or break is minimized.

[0024] When the container 10 carries a product and is fixedly held at the undulating rim 16, such as by a person's thumb and one or more fingers, the product deflects the container 10 downward relative to the fixed location and thereby exerts a tensile load on the rim 16. To alleviate the hoop stress generated by the tensile load, portions of the rim 16 near the fixed location tend to flatten, thereby increasing the length of the outer trimmed edge of the rim 16 near the fixed location. The flattened rim exhibits a localized reduction in the amplitude H to about zero and a localized increase in the wavelength λ to about 2L where the rim is flattened. As stated above, the segment length L is the distance along the surface of the wave 20 between a crest 20 a and an adjacent trough 20 b. The ratio 2L/λ corresponds to a percentage increase of the circumference along the outer trimmed edge of the rim 16 where the rim is flattened. This localized percentage increase 2L/λ ranges from about 2 percent to about 8 percent and is, most preferably, about 5 percent.

[0025]FIGS. 7a, 7 b, and 7 c depict graphs of the percentage increase 2L/λ of the circumference along the outer trimmed edge of the rim 16 where the rim is flattened for different container diameters D, amplitudes H and wave spacings α.

[0026] Referring to FIG. 7a for a container having a diameter D of 6 inches, to obtain a percentage increase of 2 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 11 mils to about 42 mils. To obtain a percentage increase of 5 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 17 mils to about 67 mils. To obtain a percentage increase of 8 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 21 mils to about 85 mils.

[0027] Referring to FIG. 7b for a container having a diameter D of 8.875 inches, to obtain a percentage increase of 2 percent, the wave spacing a is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 16 mils to about 62 mils. To obtain a percentage increase of 5 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 25 mils to about 99 mils. To obtain a percentage increase of 8 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 32 mils to about 126 mils.

[0028] Referring to FIG. 7c for a container having a diameter D of 10.25 inches, to obtain a percentage increase of 2 percent, the wave spacing a is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 18 mils to about 72 mils. To obtain a percentage increase of 5 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 29 mils to about 115 mils. To obtain a percentage increase of 8 percent, the wave spacing α is linearly varied from 2 degrees to 8 degrees as the wave amplitude H is varied from about 36 mils to about 146 mils.

[0029] In deflection tests designed to measure the reduction in rim cracks, the plastic containers 10 with the undulating rim 16 performed significantly better than conventional plastic containers without the undulating rim. For each group of tested containers, two sets of fifty plates were randomly selected for deflection testing at seven and thirty days after thermoforming. The rims of the tested containers were fixedly held by a “thumb” of a test fixture and subjected to a one-inch deflection, where the deflection is measured at the center of the container. As shown in the tables below, the plastic containers 10 embodying the present invention did not experience any rim cracks. In contrast, the conventional plastic containers experienced rim cracks, especially after the containers had aged 30 days. The rim cracks appeared under or beside the holding “thumb” of the test fixture. 50 Containers Aged 7 Days Weight Gauge 1″ Deflection Container (grams) (inches) (pounds) Rim Cracks Plain Rim 3.41 0.057 0.65 2% Undulating Rim 3.29 0.057 0.58 0%

[0030] 50 Containers Aged 30 Days Weight Gauge 1″ Deflection Container (grams) (inches) (pounds) Rim Cracks Plain Rim 3.37 0.057 0.65 42% Undulating Rim 3.28 0.059 0.60  0%

[0031] It is believed that the undulating rim 16 can better absorb the hoop stress created by a tensile load if each wave 20 of the rim 16 has a wavelength λ less than or about equal to the width of a user's thumb. In other words, when holding the container, the user's thumb should cover at least one or more waves 20. Because a small user's thumb may have a width of about 0.75 inches, the wavelength λ of each wave 20 is preferably less than or equal to about 0.75 inches. With respect to a container having a diameter D as large as 10.25 inches, this maximum wavelength λ means that the wave spacing α should be less than or equal to about 8.38 degrees based on the following equation:

[0032]  α=(360×λ)/(π×D)=(360×0.75)/(π×10.25)=8.38 degrees

[0033]

[0034] While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims. 

What is claimed is:
 1. A plastic container comprising: a bottom wall; a side wall extending upwardly and laterally outwardly from the bottom wall; and an undulating rim extending laterally outwardly from an upper portion of the side wall, the undulating rim forming a plurality of waves extending at least partially around the upper portion of the side wall, each wave forming a crest and a trough.
 2. The container of claim 1 , wherein the container is generally round.
 3. The container of claim 2 , wherein the waves occupy an entire circumference of the container.
 4. The container of claim 2 , wherein each wave occupies a circumferential angle ranging from about 2 degrees to about 8 degrees.
 5. The container of claim 1 , wherein the waves occupy an entire perimeter of the container.
 6. The container of claim 1 , wherein each wave has a wavelength λ and a segment length L equal to a distance along a surface of the wave between its crest and its trough, and wherein the ratio 2L/λ ranges from about 2 percent to about 8 percent.
 7. The container of claim 1 , wherein the crest and the trough of each wave are generally perpendicular to an adjacent section of the upper portion of the side wall.
 8. The container of claim 1 , wherein the container is a plate.
 9. The container of claim 1 , wherein each wave has a wavelength λ less than or equal to about 0.75 inches. 